1. Field of the Invention
The present invention relates to a process and apparatus for grinding a workpiece and applying an electrolytic dressing to honing stones.
2. Discussion of the Related Art
For dressing an inner surface of a cylindrical workpiece such as a cylinder bore for an automobile, a high degree of precision is required. For this purpose, honing treatment by using a honing apparatus is applied to the workpiece. By the honing treatment, it is possible to efficiently improve treatment precision, such as degrees of roundness, straightness and roughness with respect to an inner surface of a cylinder wall or the like which had been treated by fine boring and grinding.
The honing apparatus includes a honing tool including honing stones (grinding wheels) in the form of radially arranged rectangular rods. Therein, the honing stones impart a contact pressure to an inner surface of a cylindrical workpiece to grind in an external direction with respect to the diameter of the workpiece. Therefore, the workpiece is subjected to grind dressing by the reciprocating rotation of the honing tool over the entire length of the workpiece.
When a honing treatment is carried out by the honing apparatus, the honing stones rotate reciprocatingly in circumferential direction and move in a lengthwise directions of the cylindrical workpiece, simultaneously (hereinafter, referred to as rotational reciprocating movement). As a result, a ground trace in the form of a special net, that is referred to as a cross-hatch pattern, is formed on the surface of the workpiece. The cross-hatch pattern functions to store lubricating oil therein, which is necessary for a cylinder bore of an engine.
Generally speaking, each of the honing stones is in the form a rectangular rod made of extremely rigid and small grinding particles such as aluminum oxide and silicone carbide, with the particles being connected by a bonding agent. Honing stones, which have excellent autogenetic properties, that is, self-dressing properties (phenomenon of fresh grinding particles emerging after worn and flattened grinding particles fall off) while grinding a workpiece, are used.
Even with selecting the excellent honing stones, the processing precision of the workpiece varies depending on the autogenic property of the material. The autogenic cycles of the honing stones are affected, for example, by different processing preciseness of workpieces that had been treated in a previous treatment, such as fine boring and grinding, different shapes of the honing stones due to the fabrication of the honing stones, and contamination of the honing liquid used in the honing treatment.
When the autogenic phenomenon of the honing stones is not uniformly attained over the surface of the stones, it is possible that some shortcomings occur. Namely, it is possible that grinding dust/residue is pressed into gaps formed on the surfaces of the honing stones (loading/clogging), the original uneven surface of the grinding particles is worn and flattened without falling-off of the worn grinding particles (flattening/dulling), and the grinding particles fall off even with a slight grinding resistance or impact (shedding), the roughness of the treating surface is decreased, the treating surface is tempered by grinding, and the honing stones are damaged by cracking during the grinding operation. Moreover, the processing time can be excessively long lengthened. Accordingly, it is necessary to apply a dressing treatment frequently to the honing stones.
Many kinds of dressing measure for dressing stones are proposed. For example, FIG. 7 shows that a honing stone-dressing member 103 is supported at a position above a workpiece W. The dressing member 103 is in the form of a hollow cylinder and integral with an insertion guide 102 which is also in the form of a hollow cylinder. The insertion guide 102 is used for introducing a honing tool 105 in the workpiece W, so that the honing tool 105 is surrounded by the cylindrical inner surface Wa of the workpiece W. Honing treatment is applied to the workpiece W by a rotational member 101. The inner diameter of the dressing member 103 and the processing diameter of the honing stones 106 of the honing tool 105 are prepared to be approximately the same, and then the dressing stones 104 are provided inside of the dressing member 103.
The inner surface Wa of the workpiece W in the form of a cylinder is subjected to a horning treatment by using the honing tool 105. The insertion guide 102 and the dressing member 103 are caused to rotate by the rotational member 101, at an appropriate time for dressing. Then, the horning tool 105 is inserted into a dressing member 103 for thrusting out the honing stones 106 in a radial direction. Accordingly, the honing stones 16 project outwardly to contact the dressing stone 104. The honing tool 105 in this state makes a rotational reciprocating movement. It is disclosed in Japanese Kokai Publication 07 (1995)-096462 that the honing stones 106 are subjected to dressing with the honing stone 106 maintained on the honing tool 105, by the rotational reciprocating movement.
On the other hand, a metal bond wheel is frequently used, which is obtained by combining honing stones with grinding particles via an electroconductive connection made of bronze or cast iron. As dressing processes for such honing stones, various electrolytic dressing methods are proposed.
For example, FIG. 8 shows that a stone holder 112 for electrolytic dressing is provided on a honing head 111. A honing stone 115 made of grinding particles and an electroconductive connection is provided on the honing holder 112. In this state, an electrode 113 is provided closely to a processing surface, that is, an outer surface of the honing stone 115 situated at a predetermined position by the honing holder 112. The electrode 113 has a surface 113a (opposing surface 113a) having a cross section in the form of a circular arc. The opposing surface 113a opposes the outer surface of the honing stone 113 having a predetermined space therebetween. To the opposing surface 113a, an electroconductive honing liquid such as an aqueous coolant is supplied through a channel 113b. Furthermore, a voltage application member 114 is provided between the honing stone 115 and the electrode 113. By this apparatus, the application of predetermined voltage between the honing stone 115 and the electrode 113 is carried out simultaneously with the application of electroconductive honing liquid to a portion between the honing stone 115 and the electrode 113. Thus, the electrolytic dressing process is performed by electrolyzing the electroconductive connection on the outer surface of the honing stone 115 for causing the grinding particles to protrude from the outer surface of the honing stone 115. This type of electrolytic dressing method is known from Japanese Kokai Publication 2001-62721.
Japanese Patent No. 2838314 discloses a method for alternately performing electrolytic dressing and honing treatments. For carrying out this method, a workpiece 122 is provided on a rotatable chuck 121 on a processor, and a metal bond wheel 125 is provided on a shuck (not shown) which faces the workpiece 122 and makes a rotational reciprocating movement. An electrode 124 is also used, which has an electrode surface 124a facing the wheel 125, and a honing liquid supply opening 124b. A predetermined voltage is generated between the wheel 125 and the electrode 124 while the wheel 125 rotates in back and forth directions at a position between the workpiece 122 and the electrode 124. Simultaneously, an electroconductive liquid such as a coolant is supplied from the honing liquid supply opening 124b to a space between the wheel 125 and the electrode 124. In this way, electrolytic dressing and honing treatments are alternatively carried out.
Japanese Kokai Publication 07 (1995)-096462, however, has a structure that positions of the dressing member 103 and the insertion guide 102 are exchanged, for carrying out dressing of the honing stones 106 after grinding the workpieces. Therefore, an additional step, that is, to the positions of the insertion guide 102 and the dressing member 103 is required before the dressing treatment of the honing stones 106. Accordingly, the entire length of the honing cycle is increased, and hence it is possible the long operation time would affect the efficiency of the honing treatment. Moreover, it is not possible to judge the dressing state and the dressing timing without measuring the processing precision of the workpiece. Therefore, it is difficult to apply this method to a continuously operated mass-production system.
In Japanese Kokai Publication 2001-62721, it is necessary to provide a honing holder 112 to the honing head 111, and honing stones 115 to the honing holder, for dressing operation of the honing stones 115. Here, in place of the honing tool, a the honing holder 112 for electrolytic dressing is installed on the honing head 112, and the honing stones 115 detached from the honing tool is provided on the honing holder 112. Moreover, the honing stones 115, which have been subjected to a dressing treatment, are provided on the honing tool again, and the honing holder 112 detached from the honing head 111 is returned to the honing tool. Accordingly, complex operation is necessary to use the apparatus, and many operational steps are included for applying dressing treatment to the honing stones 115. That makes the honing cycle long, and the efficiency of the honing operation could be decreased.
In addition to the above, it is necessary based on the art disclosed in Japanese Patent No. 2838314 that for alternately performing electrolytic dressing and honing treatments. This alternate performance is carried out by generating a predetermined voltage between the metal bond wheel 125 and the electrode 124 during the rotational reciprocating movement of the wheel 25 between the workpiece 122 and the electrode 124, which is carried simultaneously with the supply of the electroconductive liquid such as a coolant to a space between the wheel 125 and the electrode 124. When a dressing grinding treatment is carried out by using a honing liquid with a small electroconductive or a substantially no electroconductive property, it is only possible to obtain a slight electric current in the grinding liquid, and hence it is possible that the electric current by the electrolytic dressing is unstable or non-available. Accordingly, it is possible that the electrolytic dressing capability is considerably decreased or lost.